As improved forms of protection for military targets develop, the location of a projectile or missile relative to a target becomes more significant in effective destruction of the target. Where the target is an armored vehicle, for example, the roof of the vehicle is usually the most vulnerable. Antitank missiles are designed to pass immediately over the top of armored vehicles. These missiles carry a downward firing warhead designed to penetrate the top surfaces of the target. Thus, a sensor is required which can detect the presence of a target beneath the missile and trigger the warhead at the correct moment.
No single sensor has reliably performed this task. Therefore, most concepts rely on dual sensor combinations such as magnetic-optical, microwave-optical, or magnetic-microwave. Typically, in the magnetic-optical sensor the magnetic portion is used to accurately locate the center of the target to trigger the warhead, but it is somewhat susceptible to false alarms due primarily to nearby power lines. To reduce this probability, an optical sensor is used to detect the presence of objects within the warhead damage zone beneath the missile, typically selected to be within 15 feet below. The magnetic sensor is inhibited from firing the warhead unless both the magnetic sensor and the optical sensor detect the presence of an object in the target zone.
For only a single optical sensor system in a missile 10 shown in FIG. 3, a transmitted beam 18 is aligned so that it crosses a receiver field of view 22 creating a window or sensing zone 24. If a reflective object passes through sensing zone 24, the reflected light of the transmitted beam is sensed by the receiver. If the optically impinged portion of an object passes above or below the sensing zone, the reflected light is outside the receiver field of view and is not sensed.
While this single sensor approach has worked reasonably well, over certain types of terrain and/or vegetation a double reflection occurence can cause false alarms in the optical sensor, reducing its effectiveness. Certain surfaces 51 such as deep grass terrain, even though totally below the sensing zone 24, can reflect transmitted beam light rays 52 so that some rays 52A enter the receiver field of view 22, where they can again reflect 54 up into a receiver, causing a false alarm.